scripts: add script for generating changelogs

[KanjiMonster]

Add our script for generating detailed changelogs.

Mostly identical to the latest internal MR, except the following:

• add licence and copyright
• slightly reword the help
• drop support for older bitbake versions
• add support for differences in yocto kirkstone
• allow setting version names
• ensure tmpdir always gets deleted
• dropped the single repo mode
• allow ignoring layers
• redirect tool output to STDERR instead of dropping it
• do not return a non zero exitcode when using an external workdir

Signed-off-by: Jonas Gorski jonas.gorski@bisdn.de